The curved surfaces of gears often need to be machined to a high degree of precision in order to meet design specifications. Gear measurement systems may be used to determine whether a finished gear meets the design specifications. Design specifications for gears often define the gear dimensions at the pitch circle. One gear measurement system, known as a gear checker, involves arranging a probe between two adjacent teeth of a gear along the pitch circle. The gear is then rotated until the flank of one of the gear teeth comes into contact with the probe. The gear dimension at the pitch circle can then be determined based on the distance which the gear needed to rotate in order for the tooth flank to contact the probe.
More modern gear measurement systems may employ coordinate measuring machines. With such systems, the gear is held stationary and the probe is moved into contact with the flank of a tooth of the gear. Before moving the probe, the system determines approach vectors that define the path the probe will take when moving into contact with the tooth flank at the pitch circle. The approach vectors, however, can result in the probe passing the pitch circle before the probe contacts the tooth flank. When this happens, the system can produce an incorrect value for the gear profile measurement at the pitch circle. This can lead to incorrect determinations of tooth thickness, tooth spacing and other measurements that rely upon the profile measurement at the pitch diameter.
A system that can be used to measure the curved surface of a gear is disclosed in U.S. Pat. No. 7,251,580. The system employs a coordinate measuring machine that determines a theoretical expression of the gear tooth surface, and then determines a measuring area on the tooth surface. The system defines a representative point in the measuring area, determines an axis angle of the probe and then measures the measuring area white holding the axis angle of the probe constant. The system, however, does not address the problem of the approach vector used for the probe resulting in the probe passing the pitch circle before the probe contacts the tooth flank.